Death would be certain for the newborn were it not able to use its mother's circulatory immunoglobulin for its earliest protection against disease. During fetal development or immediately after birth, maternal IgG is selectively transported into the circulatin of the fetus or neonate. The tissue responsible for the vital process varies from one genus to another, be it the placenta, mammary, yolk sac or neonatal gut. However, the cells that move the IgG from the maternal to the fetal circulation are remarkably similar. We propose to investigate the cellular and subcellular basis of this process using a chick yolk sac endothelium tissue culture system as a model system. When these cells are a confluent monolaver on callagen, the cells are morphologically very similar to the differentiated in vivo tissue. In addition, the cells bind IgG on their apical surface as the first requisite for subsequent transcellular IgG movement. Studies using the above yolk sac monolayers are proposed to: quantitate ligand binding and the kinetics of ligand internalization; identify at the ultrastructural level the vesicular route of IgG; isolate and characterize such transport vesicles; and investigate whether the receptor is recycled. The results should have implications for the entire realm of coated vesicle protein transport systems that affect cellular function.